1. Field of the Invention
The present invention relates to a reel driving clutch of magnetic recording/playback system such as a VCR (video cassette tape recorder) and DAT (digital audio cassette recorder), and more particularly to a reel driving clutch for transmitting power to a reel of the magnetic recording/playback system which uses viscous fluid, as means for transmitting power, and is contained between a disk and a reel body in order to improve the torque property and stability of the reel body and assembling simplicity of the clutch, and to miniaturize the clutch.
2. Description of the Prior Art
Referring to FIG. 1, there is shown a tape traveling system in a conventional deck of magnetic recording/playback system. In the tape traveling system, a tape 3 which is supplied from a supply reel 1 is wound around a take up reel 2 via a rotating head drum 4 by being pulled by a capstan shaft 5 and a pinch roller 6.
In the magnetic recording/playback system shown in the drawing, in order to obtain a stable picture, the capstan shaft 5 must be rotated by being in contact with the pinch roller 6 so that the tape 3 which is in contact with the rotating head drum 4 is transferred at a uniform speed.
In the tape traveling system, the linear velocity (Vc) of the tape 3 which is transferred between the capstan shaft 5 and the pinch roller 6 is expressed by the following expression: EQU Vc=Wc.times.Rc
where Rc is a radius of the capstan shaft 5 which is a fixed value and Wc is a circumferential speed of the capstan shaft 5. Accordingly, if the circumferential speed of the capstan shaft 5 is uniformly maintained, the linear velocity of the tape 3 can be uniformly maintained.
Also, the traveling linear velocity (Vt) of the tape 3 which is wound on the take up reel 2 is expressed by the following expression: EQU Vt=Wt.times.Rt
where Wt is a circumferential speed of the take up reel 2 and Rt is a variable working radius of the take up reel 2. While the circumferential speed Wt of the take up reel 2 can be controlled uniformly, the variable working radius Rt of the take up reel 2 is changed according to the amount of the tape 3 being wound on the take up reel 2.
On the other hand, since the variable working radius Rt of the take up reel 2 is changed in the range of a Rmax to a Rmin in response to the amount of the tape wound on the take up reel 2 as shown in the drawing, the linear velocity Vt of the tape 3 on the take up reel 2 is continuously changed.
Accordingly, since the linear velocity Vt of the tape 3 on the take up reel 2 is continuously changed in response to the amount of the tape 3 on the take up reel 2, the linear velocity Vt of the tape 3 on the take up reel 2 does not coincide with the linear velocity Vc on the capstan shaft 5 so that the tape 3 can not be transferred at a uniform speed.
Therefore, if the circumferential speed Wc of the capstan shaft 5 is maintained at a given speed and the circumferential speed Wt of the take up reel 2 is maintained at a slightly faster speed than the rotational speed Wc so as to transfer the tape 3 at a uniform velocity, a torque for pulling the tape 3 on the take up reel 2 becomes higher than a torque for transferring the tape 3 on the capstan shaft 5 at a fixed linear velocity as the tape 3 is wound on the take up reel 2.
At this time, since a torque difference occurs between the capstan shaft 5 and the take up reel 2, the take up reel 2 is slipped in response to the torque difference in order to compensate for the torque difference. Consequently, a linear velocity Vt of the tape on the take up reel 2 is maintained uniformly so that the tape 3 is transferred at a uniform velocity.
As mentioned above, a reel driving clutch for transmitting power to a reel is a device which causes the take up reel 2 to slip to compensate for the torque difference occurring in the tape.
Referring to FIG. 2, there is shown a conventional take up reel of a magnetic recording/playback system which has the above-mentioned reel clutch.
As shown in the drawing, the known reel driving clutch is constructed as will be described hereinafter. A bush 12 is fixedly inserted in a hole formed at a base plate 11 of magnetic recording/playback system. A shaft 13 is mounted in the bush 12. A disk 15 having an upward extending boss 15' at a central portion thereof and brake teeth 15" at its circumferential edge is rotatably mounted on the shaft 13.
The disk 15 is provided with a compression spring 16 and a washer 17 on the boss 15' thereof. Also, a support wheel 18 having gear teeth at its circumferential edge which is adapted to transmit power is rotatably mounted on the boss 15' of the disk 15 to be in contact with the washer 17.
The support wheel 18 is provided with a felt 19 at an upper surface thereof. A reel body 20 is rotatably mounted on an upper end of the boss 15' of the disk 15 in state of being in contact with the felt 19.
The reel body 20 is provided with a compression spring 21 thereon. A collar 22 is mounted on the compression spring 21. A cap 23 is rotatably mounted in a boss 20' of the reel body 20 and disposed on the collar 22. A washer 24 is fixed to the upper end of the shaft 13 and engaged with the cap 23. In the drawing, reference numeral 14 is a reflecting mirror.
Operation of the above known take up reel of the magnetic recording/playback system will be described as follows.
The take up reel 2 is driven such that the support wheel 18 is rotated by torque transmitted via an idler means adapted to transmit power (not shown). As the support wheel 18 is rotated, torque of the support wheel 18 is transmitted to the reel body 20 by being in contact with the felt 19 which is interposed between the support wheel 18 and the reel body 20 and biased upwardly by the compression spring 16. Consequently, the collar 22 mounted on the reel body 20 is rotated together with the reel body 20 so that a hub of the cassette tape is rotated by the collar 22 inserted therein, thereby allowing the tape 3 to be wound on the hub.
At this time, if the circumferential speed Wc of the capstan shaft 5 is maintained at a given speed and the circumferential speed Wt of the take up reel 2 is maintained at a slightly faster speed than the circumferential speed Wc so as to transfer the tape 3 at a uniform velocity, the tape 3 which is transferred between the capstan shaft 5 and the pinch roller 6 is normally wound on the take up reel 2 when an amount of the tape 3 wound around the take up reel 2 is small. On the other hand, when an amount of the tape 3 wound around the take up reel 2 is large, a torque for pulling the tape 3 on the take up reel 2 becomes higher than a torque for transferring the tape 3 between the capstan shaft 5 and the pinch roller 6 so that torque difference occurs in the tape 3. Accordingly, the reel body 20 slips on the felt 19, thereby causing a torque loss between the disk 15 and the reel body 20.
Therefore, since the tape 3 transferred by the capstan shaft 5 and the pinch roller 6 and the tape 3 wound around the take up reel 2 are maintained at the same linear velocity with each other, the tape 3 being in contact with the rotating head drum 4 can be transferred at the uniform linear velocity. Thus, a required function of the reel driving clutch for transmitting power to a reel can be obtained.
However, the above-mentioned known reel clutch which transmits power to the reel body 20 by a mechanical friction between the felt 19 and the reel body 20 has disadvantages as follows.
First, the felt 19 which is usually made of wool etc. is sensitive to temperature and humidity so that the felt changes highly in its physical property.
Second, since the reel driving clutch uses a transmission by friction between solid bodies, the solid bodies are severely worn down after being used for a long time, thereby causing torque stability to be decreased.
Third, it is required to have several additional components such as the felt 19, the support wheel 18, the washer 17 and the compression spring 16 in order to provide the friction between the bodies. Thus, the reel driving clutch intricate and the take up reel 2 becomes increases in height.
Fourth, it is difficult to maintain a hub guide surface of the reel body 20 at a fixed height due to physical property of the felt 19 and the spring 16. Thus, the reel driving clutch must have an additional height adjusting means.
Finally, it is difficult to machine the reel driving clutch to accommodate the felt 19 and the spring 16 and to maintain stable torque.